Install LXDE from the repository .... try using task-lxde
In the KDE session ..... Menu - Leave - Switch user
This gives me on top of screen a choice to create a New Session
Selecting this brings you to the log in screen ....... where you select LXDE from the session type provided from the icon on the bottom of the screen.
You can then log into a LXDE session, and switch between the LXDE and KDE by Ctrl,Alt+F7 (& F8)
Else you can do Menu - Leave - Switch user and select the user available.
Works well ...... but make sure you have sufficient resources to run two user sessions concurrently .... or don't do anything heavy on resources while running the two.
Forgot to mention ....... you can log in to both sessions as the same user. If you do so be sure you do not do something in one session you might regret in the other